Praying mantises are highly territorial and have large territories, they will eat each other until there is only one or two left. And, they only feed on bugs that are the size of most cannabis pests, for a small percentage of their life, quickly outgrowing there usefulness to cannabis growers.
The Praying mantis in my profile picture is too large to be useful for getting rid of anything other than grasshoppers or other mantises (and probably Ladybugs), but it is cool looking, hence the picture.
That mantis in my profile pic wandered into my garden, so I didn't pay any money for it. I love Mantids in general but, for pest control Ladybugs are more effective. I've never used Nematodes, sounds like a fun experiment.
Good luck.